Tag: OSPF

Intermediate System to Intermediate System (IS-IS)

Intermediate System to Intermediate System (IS-IS) is a dynamic link-state routing protocol widely used in large-scale networks, especially in service provider environments. In this post I will look at the following IS-IS configurations;

• Redistribution of OSPF into IS-IS, and the differences between L1 and L2.
• IS-IS Path selection when L1 and L2 routes are available.
• How and where to apply IS-IS Route Summarization.

IS-IS Lab Setup

Using Cisco’s Modeling Labs (CML) I build the following IS-IS and OSPF topology:

• 2 x L2 Router ( Top )  (R1, R2) running XRv with IOS-XR.
• 2 x L1L2 Router ( left ) (R3, R4) running XRv with IOS-XR.
• 2 x L1 Router ( left ) (R7, R8) running IOS with IOSv.
• 2 x L1L2 Router ( Center ) (R5, R6) running XRv with IOS-XR.
• 2 x L1 Router ( Center ) (R9, R10) running IOS with IOSv.
• 1 x OSPF Router (R11) running IOS with IOSv.

Logical View:

Physical View:

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What is MPLS Segment Routing (MPLS-SR)?

In short MPLS Segment Routing (MPLS-SR) is a modern approach to routing in MPLS (Multiprotocol Label Switching) networks. It allows for the efficient steering of traffic through predefined network segments. These segments are advertised by link-state routing protocols (IS-IS, OSPFv2, and OSPFv3) within IGP topologies. With MPLS-SR, path control and traffic engineering can be achieved without the need for protocols like LDP or RSVP-TE, which are typically used to set up traffic-engineered paths in traditional MPLS networks. These segments are stacked as labels in packet headers, allowing routers to follow predefined paths for traffic without this additional state tracking. This simplifies traffic engineering, enhances scalability, and enables efficient routing.

In this lab I will demonstrate the process of migrating from a traditional MPLS setup (using MPLS, OSPF, and LDP) to an MPLS-SR configuration on IOS-XR and IOS-XE. In this guide, you will see the configuration steps required and differences between both operating systems.
Following this, I will configure a Segment Routing Mapping Server (SRMS) to map the prefixes of IOSv routers since they don’t support MPLS-SR.

MPLS Lab Setup (Baseline)

Using Cisco’s Modeling Labs (CML) I build the following MPLS lab using OSPF and LDP neighbor relationships. 

• 2 x PE router ( Left ) (PE5, PE6) running CSR1000v with IOS-XE.
• 4 x P router ( Center )  (P1, P2, P3, P4) running XRv with IOS-XR.
• 2 x PE router ( Right ) (PE7, PE8) running IOSv with IOS.

Logical View:

Interfaces:

Firmware:

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Setup:

• Dual-homed BGP between AS100 and AS200
• AS100
  • HSRP 192.168.0.10 between R1 and R2
  • Router 1 HSRP Master
  • Linkstate tracking on Fa0/0
  • EIGRP as IGP
• AS200
  • HSRP 10.10.10.10 between R3 and R4
  • Router 3 as HSRP Master
  • Linkstate tracking on Fa0/0
  • OSPF for IGP

Scenario: The link between Router1 and Router3 would fail. Linkstate tracking would decrement the HSRP priority and switch masters.

When the link was restored and using default HSRP timers, the HSRP master would switch back before the BGP session was established between Router1 and Router3 (at least in GNS3).
Setting up delay timers and linkstate tracking would allow for a good recovery.

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